package gnu.crypto.mode;
// ----------------------------------------------------------------------------
// $Id: ICM.java,v 1.7 2005/10/06 04:24:17 rsdio Exp $
//
// Copyright (C) 2001, 2002, 2003 Free Software Foundation, Inc.
//
// This file is part of GNU Crypto.
//
// GNU Crypto is free software; you can redistribute it and/or modify
// it under the terms of the GNU General Public License as published by
// the Free Software Foundation; either version 2, or (at your option)
// any later version.
//
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// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
// General Public License for more details.
//
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// along with this program; see the file COPYING. If not, write to the
//
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// USA
//
// Linking this library statically or dynamically with other modules is
// making a combined work based on this library. Thus, the terms and
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// do so, delete this exception statement from your version.
// ----------------------------------------------------------------------------
import gnu.crypto.Registry;
import gnu.crypto.cipher.IBlockCipher;
import java.math.BigInteger;
/**
* <p>An implementation of <i>David McGrew</i> Integer Counter Mode (ICM) as an
* {@link IMode}.</p>
*
* <p>ICM is a way to define a pseudorandom keystream generator using a block
* cipher. The keystream can be used for additive encryption, key derivation,
* or any other application requiring pseudorandom data. In the case of this
* class, it is used as additive encryption, XOR-ing the keystream with the
* input text --for both encryption and decryption.</p>
*
* <p>In ICM, the keystream is logically broken into segments. Each segment is
* identified with a segment index, and the segments have equal lengths. This
* segmentation makes ICM especially appropriate for securing packet-based
* protocols. ICM also allows a variety of configurations based, among other
* things, on two parameters: the <i>block index length</i> and the
* <i>segment index length</i>. A constraint on those two values exists: The sum
* of <i>segment index length</i> and <i>block index length</i> <b>must not</b>
* half the <i>block size</i> of the underlying cipher. This requirement protects
* the ICM keystream generator from potentially failing to be pseudorandom.</p>
*
* <p>For simplicity, this implementation, fixes these two values to the
* following:</p>
*
* <ul>
* <li>block index length: is half the underlying cipher block size, and</li>
* <li>segment index length: is zero.</li>
* </ul>
*
* <p>For a 128-bit block cipher, the above values imply a maximum keystream
* length of 295,147,905,179,352,825,856 octets, since in ICM, each segment must
* not exceed the value <code>(256 ^ <i>block index length</i>) * <i>block length</i></code>
* octets.</p>
*
* <p>Finally, for this implementation of the ICM, the IV placeholder will be
* used to pass the value of the <i>Offset</i> in the keystream segment.</p>
*
* <p>References:</p>
*
* <ol>
* <li><a href="http://www.ietf.org/internet-drafts/draft-mcgrew-saag-icm-00.txt">
* Integer Counter Mode</a>, David A. McGrew.</li>
* </ol>
*
* @version $Revision: 1.7 $
*/
public class ICM extends BaseMode implements Cloneable {
// Constants and variables
// -------------------------------------------------------------------------
/** The integer value 256 as a BigInteger. */
private static final BigInteger TWO_FIFTY_SIX = new BigInteger("256");
/** Maximum number of blocks per segment. */
private BigInteger maxBlocksPerSegment;
/** A work constant. */
private BigInteger counterRange;
/** The initial counter for a given keystream segment. */
private BigInteger C0;
/** The index of the next block for a given keystream segment. */
private BigInteger blockNdx;
// Constructor(s)
// -------------------------------------------------------------------------
/**
* <p>Trivial package-private constructor for use by the Factory class.</p>
*
* @param underlyingCipher the underlying cipher implementation.
* @param cipherBlockSize the underlying cipher block size to use.
*/
ICM(IBlockCipher underlyingCipher, int cipherBlockSize) {
super(Registry.ICM_MODE, underlyingCipher, cipherBlockSize);
}
/**
* <p>Private constructor for cloning purposes.<p>
*
* @param that the instance to clone.
*/
private ICM(ICM that) {
this((IBlockCipher) that.cipher.clone(), that.cipherBlockSize);
}
// Class methods
// -------------------------------------------------------------------------
// Cloneable interface implementation
// -------------------------------------------------------------------------
public Object clone() {
return new ICM(this);
}
// Implementation of abstract methods in BaseMode
// -------------------------------------------------------------------------
public void setup() {
if (modeBlockSize != cipherBlockSize) {
throw new IllegalArgumentException();
}
counterRange = TWO_FIFTY_SIX.pow(cipherBlockSize);
maxBlocksPerSegment = TWO_FIFTY_SIX.pow(cipherBlockSize / 2);
BigInteger r = new BigInteger(1, iv);
C0 = maxBlocksPerSegment.add(r).modPow(BigInteger.ONE, counterRange);
blockNdx = BigInteger.ZERO;
}
public void teardown() {
counterRange = null;
maxBlocksPerSegment = null;
C0 = null;
blockNdx = null;
}
public void encryptBlock(byte[] in, int i, byte[] out, int o) {
icm(in, i, out, o);
}
public void decryptBlock(byte[] in, int i, byte[] out, int o) {
icm(in, i, out, o);
}
// Instance methods
// -------------------------------------------------------------------------
private void icm(byte[] in, int inOffset, byte[] out, int outOffset) {
if (blockNdx.compareTo(maxBlocksPerSegment) >= 0)
throw new RuntimeException("Maximum blocks for segment reached");
// encrypt the counter for the current blockNdx
// C[i] = (C[0] + i) modulo (256^BLOCK_LENGTH).
BigInteger Ci = C0.add(blockNdx).modPow(BigInteger.ONE, counterRange);
byte[] result = Ci.toByteArray();
int limit = result.length;
// if (limit < cipherBlockSize) {
// byte[] data = new byte[cipherBlockSize];
// System.arraycopy(result, 0, data, cipherBlockSize-limit, limit);
// result = data;
// } else if (limit > cipherBlockSize) {
// byte[] data = new byte[cipherBlockSize];
// System.arraycopy(result, limit-cipherBlockSize, data, 0, cipherBlockSize);
// result = data;
// }
//
// cipher.encryptBlock(result, 0, result, 0);
// blockNdx = blockNdx.add(BigInteger.ONE); // increment blockNdx
// for (int i = 0; i < modeBlockSize; ) { // xor result with input block
// out[outOffset++] = (byte)(in[inOffset++] ^ result[i++]);
// }
int ndx = 0;
if (limit < cipherBlockSize) {
byte[] data = new byte[cipherBlockSize];
System.arraycopy(result, 0, data, cipherBlockSize-limit, limit);
result = data;
} else if (limit > cipherBlockSize) {
ndx = limit-cipherBlockSize;
}
cipher.encryptBlock(result, ndx, result, ndx);
blockNdx = blockNdx.add(BigInteger.ONE); // increment blockNdx
for (int i = 0; i < modeBlockSize; i++) { // xor result with input block
out[outOffset++] = (byte)(in[inOffset++] ^ result[ndx++]);
}
}
}